The present invention relates generally to photovoltaic (PV) power systems and, more particularly, to a PV array that incorporates a set of contactors that are selectively operated in order to provide an increased working voltage for a PV inverter in the PV power system without the use of DC-DC converters.
PV power systems are power systems that employ a plurality of solar modules to convert sunlight into electricity. PV systems include multiple components, including photovoltaic modules, mechanical and electrical connections and mountings, and means of regulating or modifying the electrical output. One common arrangement in PV systems is for several PV modules to be connected in series to form a PV string, with multiple PV strings in a PV system then being combined in parallel to aggregate the current in a PV array. The output of multiple PV arrays can then be combined by way of a combiner box that aggregates the DC power from the PV arrays and provides a connection point (i.e., a common bus) for the PV arrays.
In operation, PV modules generate direct current (DC) power, with the level of DC current being dependent on solar irradiation and the level of DC voltage dependent on temperature. When alternating current (AC) power is desired, an inverter is used to convert the DC energy from the array into AC energy, such as AC energy suitable for transfer to a power grid. Typical PV inverters employ two stages for power processing. The first stage of the PV inverter is configured to regulate a widely varying DC voltage from an array of PV cells, so as to provide a constant DC voltage output. The second stage of the PV inverter is configured to convert the constant DC voltage to AC current. Often, the first stage includes a boost converter, and the second stage includes a single-phase or three-phase inverter system.
For converting the varying DC voltage of a PV array to the fixed frequency AC voltage of the power grid, many PV inverters employ a two-stage conversion power circuit that uses a DC link as an intermediate energy storage step, which means that the converter first converts the varying PV array voltage to a stable DC voltage. The PV DC-to-AC inverter then subsequently converts the stable voltage into an AC current that can be injected into the grid. Alternatively, PV inverters can instead employ a single stage conversion power circuit in which a transformer is employed to boost the AC voltage.
With respect to typical two stage PV inverters, one drawback is that such inverters are inherently less efficient and more costly due to the second stage. That is, the efficiency of the two-stage inverter is a multiple of the individual stage efficiencies, with each stage typically causing one-half of the system losses. It would thus be beneficial to eliminate one stage of the PV inverter, i.e., eliminate the DC-DC converter, so as to increase efficiency of the inverter. However, it is recognized that elimination of the DC-DC converter would result in the inverter having a smaller DC operating window, as single stage PV inverters are recognized as having a smaller DC voltage operating window. Thus, in eliminating the DC-DC converter, a mechanism is needed that minimizes variation of the output of the PV array(s) to the PV inverter.
Another recognized drawback with existing PV array and PV inverter arrangements is that the DC power and voltage generated by the PV array groups can widely vary, as the amount of DC power/voltage generated thereby is dependent on the temperature at which PV arrays operate and the amount of solar irradiation received by the PV arrays. Accordingly, the PV inverter may not always be operating at an optimal working voltage if the amount of DC voltage received from the PV array is less than the optimal working voltage level. This can occur during periods of cloudy weather or early/late in the day, as the level of solar irradiation received by the PV array may not be adequate at these times. It would be beneficial if the variation of the DC voltage received by the PV inverter from the PV array could be minimized and that sufficient DC voltage could be provided from the PV array to allow the PV inverter to operate at or close to an optimal working voltage, such that the PV inverter operates at a higher or peak efficiency.
It would therefore be desirable to provide a PV array to a PV inverter that employs a single stage topology, as compared to a traditional two-stage PV inverter, so as to provide an increased working voltage and minimize system losses, while still providing the larger DC operating window of a two-stage design. It would also be desirable to provide a PV inverter, and technique for controlling thereof, that minimizes variation of the output of the PV array(s) to the PV inverter and that ensures the PV inverter is always operating at peak efficiency.